Oil burning submerged combustion units

ABSTRACT

In an oil burning combustion unit a mixture of oil and combustion air is directed downwards into a combustion chamber submerged in a liquid, and having a step at which its crosssection increases in a downward direction. It is made from a single thickness of metal with its inner surface exposed to the flame and its outer surface immersed in the liquid.

United States Patent Booth 1 July 24, 1973 01L BURNING SUBMERGED COMBUSTION 3,138,150 6/1964 Hyeret al. 126/360 UNITS 2,677,368 /1554 Janecek 2,900,975 8/1959 Northcott 126/360 [75] Inventor: Christopher John Booth, Chester,

England Prima Examiner-Kenneth W. S" ra ue [73] Assignee: C. & W. Walker Limited, ry P g Donnington near wellington, Attorney-Wats0n, Cole, Grindle & Watson Shropshire, Great Britain [22] Filed: June 10, 1971 [2]] Appl. No.: 151,881 [57] ABSTRACT In an oil burning combustion unit a mixture of oil and 1 0 126/350 R combustion air is directed downwards into a combus- [51] Int. Cl. F24h 1/20 tion chamber submerged in a liquid, and having a step, [58] Field of Search 60/39.55, 39.57; at which its cross-section increases in a downward di- 360 3 rection. It is made from a single thickness of metal with its inner surface exposed to the flame and its outer sur- [56] References Cited face immersed in the liquid.

UNITED STATES PATENTS I 2,810,382 /1957 Warren 126/360 18 Claims, 1 Drawing Figure Bx-d 77 33 OIL BURNING SUBMERGED COMBUSTION UNITS This invention relates to submerged combustion units, and is concerned with the provision of a submerged combustion unit capable fo satisfactorily burning oil.

According to the present invention an oil'burning combustion unit includes an oil atomizer for directing atomized oil and combustion air downwards into a combustion chamber with an opening at the bottom and submerged in a liquid such as water, in which the combustion chamber has in it a step at which its crossseetion increases suddenly in a downward direction, the wall of the chamber both above and below the step being formed of a single thickness of metal with its inner surface exposed to the flame and its outer surface immersed in the liquid.

Preferably the combustion chamber is of circular section and the enlarged portion of it, below the step, has a length between half and one and a half times its diameter.

Thus in one form of the invention the combustion chamber has an upper diverging portion whose diameter increases progressively downwards, a middle cylindrical portion of a substantially constant diameter equal to that of the lower end of the upper portion extending from its to the step, and a lower cylindrical portion of substantially constant diameter extending down from the step of the open end.

For heating a liquid submerged combustion has considerable advantages in that with a combustion chamber below the surface of the liquid, and therefore in intimate contact with it, and with the exhaust gases passing from the combustion chamber directly into the liquid in which the burner is submerged, efficient transfer of heat is obtained in a simple manner. Moreover it does not require a refractory lining, resulting in a saving in capital outlay, and indeed could not employ one since the hot refractory material would be shattered when flooded with liquid on shutting off the burner.

Although oil is a cheaper fuel than gas, therm for therm, it has hitherto been customery to employ gas because of its ease or ignition and stability of flame, and difficulty has been experienced in obtaining satisfactory results with oil. Thus whereas a gas flame is a continuous medium and burns as a single continuous flame, oil is atomized in its combustion air and is therefore a mist of tiny droplets each of which burns and each of which forms a very small flame within the combustion zone, nd each of which has to be ignited. These droplets burn at their surfaces where they are evaporating and the oil and air are truly mixed at a molecular scale. Thus the function of an oil bruner is first to break up the oil into small droplets, secondly to evaporate these droplets and thirdly to burn the evaporated droplets as a gas so as to evolve heat part of which serves to evaporate incoming droplets.

Considerable difficulty is experienced in obtaining a reliable and stable flame with oil fuel. in a conventional combustion chamber the incoming oil drops are vapourized by heat radiated from the walls of the combustion chamber which are incandescent. This necessitates the employment of a refractory lining which adds substantially to the cost and also reduces the transfer of heat. It is believed that the step in a combustion chamber in accordance with the present invention introduces a degree of turbulence which results in a satisfactory transfer of heat from the burning vapourized oil to the incoming oil droplets, and produces a reliable and stable flame.

The present invention may be put into practice in various ways but one specific embodiment will now be described by way of example with reference to the accompanying drawing which is a somewhat diagrammatic elevation, partly in section, of an oil-burning submerged combustion unit for heating water for purposes such as swimming baths.

The unit comprises an oil-burning atomizer 10 of conventional type directed downwardly into a combustion chamber 14. The combustion chamber 14 is formed of a single thickness of sheet steel and comprises an upper frusto-conical portion 15 diverging downwardly from the oil atomizer 10, a cylindrical middle portion 16 of the same diameter as the lower larger end of the upper portion 15, and a horizontal step 17 connecting the lower end of the cylindrical portion to the upper end of a lower cylindrical portion 18 of larger diameter than the middle portion 16. A flat circular plate 19 is connected to, and spaced from, the lower end of the lower cylindrical portion 18 by four straps 20.

It is found that there is a critical minimum length for the lower portion 18 of the combustion chamber 14, of which the ratio of length to diameter should lie between 1.5 and 0.5. Thus in the specific embodiment the proportions of the parts are roughly as follows, taking the diameter of the lower open end of the lower portion 18 as one. On this basis the height of the upper portion 15, the middle portion 16 and the lower portion 18 are each 0.64, 1.0 and 1.0 respectively. The diameter of the middle portion 16 and of the lower end of the upper portion 15 is 0.67 and the diameter of the upper end of the upper portion 15 is about 0.33.

The combustion chamber is secured to a flat plate 25 and the water level may vary between the limits indicated by the lines 26 and 27, which are respectively, on the basis of the above proportions, 0.] below and 1.0 above the flat plate 25.

With these proportions it is found that a stable flame can be produced with the water in contact with the outer surface of the combustion chamber 14 and the flame in contact with its inner surface. Due to the water-cooling the wall is not destroyed and at the same time efficient transfer of heat occurs. COnveniently the oil burner is provided with a pilot gas burner 30 to ignite it. This comprises a smaller cylindrical combustion chamber 31 mounted vertically alongside the oilburning unit having at the top an inlet tube 32, through which a mixture of gas and air is admitted to the gas burner, a small opening 33 in the bottom and a lateral port 34 opening into the combustion chamber 14 of the oil-burning unit. A flash tube 37 extends from the gas combustion chamber 31 to a point above the surface of the water and is provided with a sparking plug 38 and venting valve 39.

The oil combustion chamber 14 and the gas combustion chamber 31 may be provided with ultra-violet flame detectors, 42 and 43 respectively, to automatically shut off the supplies of oil and gas if the flames are extinguished.

Thus to start the oil burner air from blowers 50 and 51 is admitted to the combustion chambers 14 and 31 to completely purge the system. Gas is then admitted from a supply 52 to the gas combustion chamber 31, by

way of a venturi device 53 and the inlet tube 32, in combustible proportions with air and the venting valve 39 opened to allow some gas to flow up the flash tube 37 and be ignited, by the sparking plug 38, to start the gas flame. The rate of flow of the inflammable mixture up the flame tube is governed by a restriction 40, which may be incorporated in the valve 39, so that on ignition the flame will flash back into the gas combustion chamber 31. The oil is then turned on and the gas flame, which extends through the port 34 into the oil combustion chamber 14, ignites it. If desired the the gas flame may now be turned off provided that either a good seal is made at the top of the gas burner or air is left flowing through this part of the system.

The hole 33 in the base of the gas burner 30 is desirable to avoid detonation on ignition of the gas and consequent damage to air compression equipment.

The preferred gas is propane but other gaseous fuels preferably with a calorific value above 350 BTU/ft may be used.

What I claim as our invention and desire to secure by Letters Patent is:

1. An oil burning combustion unit having an oil atomizer operable to direct atomized oil and combustion air downwardly into a combustion chamber;

said combustion chamber having an opening in the lower end thereof, being substantially circular in horizontal cross-section and substantially submerged in a liquid such as water in operation;

said combustion chamber further having a stepped construction where its horizontal cross-section increases suddenly in a downward direction, the enlarged portion of said combustion chamber below the step having a length between A and 1% times the diameter thereof;

said combustion chamber further having an upper diverging portion whose diameter increases progressively downwardly, a middle substantially cylindrical portion having a substantially constant diameter substantially equal to that of the lower end of said upper diverging portion and extending from same to the step, the lower cylindrical portion of said combustion chamber from said step downwardly to the open end thereof having substantially constant diameter;

the opening at the lower end of said combustion chamber being of substantially annular form and defined by a substantially circular plate spaced downwardly from the otherwise substantially circular open lower end of the combustion chamber wall below the step therein.

2. A unit as in claim 1 including a pilot gas burner provided in conjunction with said unit and mounted in substantial axial alignment with the combustion chamber of said unit, said pilot gas burner having a port connccting the combustion chamber of the pilot gas burner to the middle substantially cylindrical portion of said unit combustion chamber above the step therein, whereby to ignite the flame in the unit combustion chamber.

3. A unit as in claim 1 wherein the unit combustion chamber has an ultraviolet flame detector and means associated therewith operative to shut off the supply of oil to the oil atomizer should the flame be extinguished in the unit combustion chamber.

4. A unit as in claim 1 wherein the proportions of the parts of the unit combustion chamber, taking the diameter of the opening at the lower end thereof as 1.0, are:

diameter of the unit combustion chamber above the step is 0.67 and the diameter of the upper end of the unit combustion chamber upper portion is 0.33, the height of the lower portion of the unit combustion chamber below the step is 1.0, the height of the middle portion of the combustion chamber above the step is 1.0 and the height of the upper portion of the unit combustion chamber is 0.64.

5. An oil burning combustion unit having an oil atomizer operable to direct atomized oil and combustion air downwardly into a combustion chamber;

said combustion chamber having an opening in the lower end thereof, being substantially circular in horizontal cross-section and in operation substantially submerged in a liquid such as water;

said combustion chamber having an uppermost diverging portion whose diameter increases progressively downwardly, a middle cylindrical portion of a substantially constant diameter equal to that of the lower end of the uppermost portion and extending from the latter to a step, and a lower cylindrical portion of increased diameter extending downwardly from said step to the open end;

the wall of the unit combustion chamber above and below said step formed of a single thickness of metal whereby the inner surface thereof is exposed to the flame and the outer surface thereof is in direct contact with said liquid in operation.

6. A unit as in claim 5 wherein the lowermost large diameter portion of said unit combustion chamber, below the step, has a length between k and 1% times its diameter.

7. A unit as in claim 5 wherein the outlet opening from the burner to the surrounding fluid in operation is defined in annular form by a substantially circular plate downwardly spaced from the otherwise open circular lower end of the unit combustion chamber wall.

8. An oil burning combustion unit having an oil atomizer operable to direct atomized oil and combustion air downwardly into a combustion chamber;

said combustion chamber having a discharge opening at the lower end thereof, being substantially circular in horizontal cross-section throughout the substantial height thereof and substantially submerged in a liquid such as water during operation;

said combustion chamber further of substantial two part construction vertically separated by a step whereat the diameter thereof increases suddenly downwardly;

the enlarged portion of said unit combustion chamber, below the step therein, having a length between k and 1% times the diameter thereof;

the wall of the unit combustion chamber and both above and below the step formed of a single thickness of metal whereby the inner surface thereof is exposed to the flame, the outer surface thereof in direct contact with the liquid in which it is submerged in operation.

9. An oil burning combustion unit having an oil atomizer operable to direct atomized oil and combustion air downwardly into a combustion chamber;

said combustion chamber having a discharge opening in the lower end thereof, having the substantial same configuration to shape in or entire vertical height thereof in horizontal section therethrough and substantially submerged in a liquid such as water in operation thereof;

said combustion chamber further of a step construction where its cross-sectional dimension increases suddenly in a downward direction, the enlarged portion F said combustion chamber below the step having a length between 1% and 1% times the diameter thereof;

the wall of said chamber both above and below the said step formed of a single thickness of metal whereby the inner surface thereof is exposed to the flame and the outer surface thereof is in direct contact with the liquid in which same is submerged in operation.

10. An oil burning combustion unit having an oil atomizer operable to direct atomized oil and combustion air downwardly into a combustion chamber;

said combustion chamber having a discharge opening in the lower end thereof, of the same configuration or shape in horizontal cross-section throughout the vertical height thereof and substantially submerged in a liquid such as water in operation;

said combustion chamber having an uppermost diverging portion whose diameter increases progressively downwardly, a middle portion of a substantially constant diameter (or transverse dimension) equal to that of the lower end of said uppermost portion and extending downwardly therefrom to a step, at the step the cross-sectional dimension of the combustion chamber increasing suddenly in a downward direction and a lowermost portion of substantially constant diameter or transverse dimension extending downwardly from the step to the open end thereof;

the wall of said chamber both above and below the said step formed of a single thickness of metal whereby the inner surface thereof is exposed to the flame and the outer surface thereof is in direct contact with the liquid in which same is submerged in operation.

11. An oil burning combustion unit including an oil atomizer for directing atomized oil and combustion air downwards into a combustion chamber with an opening at the bottom and submerged in a liquid such as water, in which the combustion chamber has in it a step at which its cross-section increases suddenly in a downward direction, the wall of the chamber both above and below the step being formed of a single thickness of metal with its inner surface exposed to the flame and its outer surface immersed in the liquid, the combustion chamber is of circular section and the enlarged portion of it, below the step, has a length between 5% and W2 times its diameter, the combustion chamber also having an upper diverging portion whose diameter increases progressively downwards, a middle cylindrical portion of a substantially constant diameter equal to that of the lower end of the upper portion extending from it to the step, and a lower cylindrical portion of substantially constant diameter extending down from the step to the open end.

' 12. An oil burning combustion unit including an oil atomizer for directing atomized oil and combustion air downwards into a combustion chamber with an opening at the bottom and submerged in a liquid such as water, in which the combustion chamber has in it a step at which its cross-section increases suddenly in a downward direction, the wall of the chamber both above and below the step being formed of a singlethickness of metal with its inner surface exposed to the flame and its outer surface immersed in the liquid, the combustion chamber is of circular section, the enlarged portion of it, below the step, has a length between A and 1% times its diameter, and the opening at the bottom of the combustion chamber is of annular form provided by a plate spaced from the otherwise open lower end of the combustion chamber wall.

13. An oil burning combustion unit including an oil atomizer for directing atomized oil and combustion air downwards into a combustion chamber with an opening at the bottom and submerged in a liquid such as water, in which the combustion chamber has in it a step at which its cross-section increases suddenly in a downward direction, the wall of the chamber both above and below the step being formed of a single thickness of metal with its inner surface exposed to the flame and its outer surface immersed in the liquid, the combustion chamber is of circular section, the enlarged portion of it, below the step, has a length between A and 1% times its diameter, and a pilot gas burner, mounted alongside and having a port connecting it to the combustion chamber, which ignites the flame.

14. An oil burning combustion unit including an oil atomizer for directing atomized oil and combustion air downwards into a combustion chamber with an opening at the bottom and submerged in a liquid such as water, in which the combustion chamber has in it a step atwhich its cross-section increases suddenly in a downward direction, the wall of the chamber both above and below the step being formed of a single thickness of metal with its inner surface exposed to the flame and its outer surface immersed in the liquid, the combustion chamber is of circular section, the enlarged portion of it, below the step, has a length between 1% and 1% times its diameter, and the combustion chamber provided with an ultraviolet flame detector which shuts off the supply of oil if the flame is extinguished.

15. An oil burning combustion unit including an oil atomizer for directing atomized oil and combustion air downwards into a combustion chamber with an opening at the bottom and submerged in a liquid such as water, in which the combustion chamber has in it a step at which its cross-section increases suddenly in a downward direction, the wall of the chamber both above and below the steo being formed of a single thickness of metal with its inner surface exposed to the flame and its outer surface immersed in the liquid, and the combustion chamber has an upper diverging portion whose diameter increases progressively downwards, middle cylindrical portion of a substantially constant diameter equal to that of the lower end of the upper portion extending from it to the step, and a lower cylindrical portion of substantially constant diameter extending down from the step to the open end.

16. An oil burning combustion unit including an oil atomizer for directing atomized oil and combustion air downwards into a combustion chamber with an opening at the bottom and submerged in a liquid such as water, in which the combustion chamber has in it a step at which its cross-section increases suddenly in a downward direction, the wall of the chamber both above and below the step being formed of a single thickness of metal with its inner surface exposed to the flame and its outer surface immersed in the liquid, and the opening at the bottom of the combustion chamber, is of annular form provided by a plate spaced from the otherwise open lower end of the combustion chamber wall.

17. An oil burning combustion unit including an oil atomizer for directing atomized oil and combustion air downwards into a combustion chamber with an opening at the bottom and submerged in a liquid such as water, in which the combustion chamber has in it a step at which its cross-section increases suddenly in a downward direction, the wall of the chamber both above and below the step being formed of a single thickness of metal with its inner surface exposed to the flame and its outer surface immersed in the liquid, and a pilot gas burner, mounted alongside and having a port connecting it to the combustion chamber, which ignites the flame.

18. An oil burning combustion unit including an oil atomizer for chamber atomized oil and combustion air downwards into a combustion ,hamber with an opening at the bottom and submerged in a liquid such as water, in which the combustion chamber has in it a step at which its cross-section increases suddenly in a downward direction, the wall of the chamber both above and below the step being formed of a single thickness of metal with its inner surface exposed to the flame and its outer surface immersed in the liquid, and the combustion chamber provided with an ultraviolet flame detector which shuts off the supply of oil if the flame is extinguished. 

1. An oil burning combustion unit having an oil atomizer operable to direct atomized oil and combustion air downwardly into a combustion chamber; said combustion chamber having an opening in the lower end thereof, being substantially circular in horizontal crosssection and substantially submerged in a liquid such as water in operation; said combustion chamber further having a stepped construction where its horizontal cross-section increases suddenly in a downward direction, the enlarged portion of said combustion chamber below the step having a length between 1/2 and 1 1/2 times the diameter thereof; said combustion chamber further having an upper diverging portion whose diameter increases progressively downwardly, a middle substantially cylindrical portion having a substantially constant diameter substantially equal to that of the lower end of said upper diverging portion and extending from same to the step, the lower cylindrical portion of said combustion chamber from said step downwardly to the open end thereof having substantially constant diameter; the opening at the lower end of said combustion chamber being of substantially annular form and defined by a substantially circular plate spaced downwardly from the otherwise substantially circular open lower end of the combustion chamber wall below the step therein.
 2. A unit as in claim 1 including a pilot gas burner provided in conjunction with said unit and mounted in substantial axial alignment with the combustion chamber of said unit, said pilot gas burner having a port connecting the combustion chamber of the pilot gas burner to the middle substantially cylindrical portion of said unit combustion chamber above the step therein, whereby to ignite the flame in the unit combustion chamber.
 3. A unit as in claim 1 wherein the unit combustion chamber has an ultraviolet flame detector and means associated therewith operative to shut off the supply of oil to the oil atomizer should the flame be extinguished in the unit combustion chamber.
 4. A unit as in claim 1 wherein the proportions of the parts of the unit combustion chamber, taking the diameter of the opening at the lower end thereof as 1.0, are: diameter of the unit combustion chamber above the step is 0.67 and the diameter of the upper end of the unit combustion chamber upper portion is 0.33, the height of the lower portion of the unit combustion chamber below the step is 1.0, the height of the middle portion of the combustion chamber above the step is 1.0 and the height of the upper portion of the unit combustion chamber is 0.64.
 5. An oil burning combustion unit having an oil atomizer operable to direct atomized oil and combustion air downwardly into a combustion chamber; said combustion chamber having an opening in the lower end thereof, being substantially circular in horizontal cross-section and in operation substantially submerged in a liquid such as water; said combustion chamber having an uppermost diverging portion whose diameter increases progressively downwardly, a middle cylindrical portion of a substantially constant diameter equal to that of the lower end of the uppermost portion and extending from the latter to a step, and a lower cylindrical portion of increased diameter extending downwardly from said step to the open end; the wall of the unit combustion chamber above and below said step formed of a single thickness of metal whereby the inner surface thereof is exposed to the flame and the outer surface thereof is in direct contact with said liquid in operation.
 6. A unit as in claim 5 wherein the lowermost large diameter portion of said unit combustion chamber, below the step, has a length between 1/2 and 1 1/2 times its diameter.
 7. A unit as in claim 5 wherein the outlet opening from the burner to the surrounding fluid in operation is defined in annular form by a substantially circular plate downwardly spaced from the otherwise open circular lower end of the unit combustion chamber wall.
 8. An oil burning combustion unit having an oil atomizer operable to direct atomized oil and combustion air downwardly into a combustion chamber; said combustion chamber having a discharge opening at the lower end thereof, being substantially circular in horizontal cross-section throughout the substantial height thereof and substantially submerged in a liquid such as water during operation; said combustion chamber further of substantial two part construction vertically separated by a step whereat the diameter thereof increases suddenly downwardly; the enlarged portion of said unit combustion chamber, below the step therein, having a length between 1/2 and 1 1/2 times the diameter thereof; the wall of the unit combustion chamber and both above and below the step formed of a single thickness of metal whereby the inner surface thereof is exposed to the flame, the outer surface thereof in direct contact with the liquid in which it is submerged in operation.
 9. An oil burning combustion unit having an oil atomizer operable to direct atomized oil and combustion air downwardly into a combustion chamber; said combustion chamber having a discharge opening in the lower end thereof, having the substantial same configuration ro shape in or entire vertical height thereof in horizontal section therethrough and substantially submerged in a liquid such as water in operation thereof; said combustion chamber further of a step construction where its cross-sectional dimension increases suddenly in a downward direction, the enlarged portion oF said combustion chamber below the step having a length between 1/2 and 1 1/2 times the diameter thereof; the wall of said chamber both above and below the said step formed of a single thickness of metal whereby the inner surface thereof is exposed to the flame and the outer surface thereof is in direct contact with the liquid in which same is submerged in operation.
 10. An oil burning combustion unit having an oil atomizer operable to direct atomized oil and combustion air downwardly into a combustion chamber; said combustion chamber having a discharge opening in the lower end thereof, of the same configuration or shape in horizontal cross-section throughout the vertical height thereof and substantially submerged in a liquid such as water in operation; said combustion chamber having an uppermost diverging portion whose diameter increases progressively downwardly, a middle portion of a substantially constant diameter (or transverse dimension) equal to that of the lower end of said uppermost portion and extending downwardly therefrom to a step, at the step the cross-sectional dimension of the combustion chamber increasing suddenly in a downward direction and a lowermost portion of substantially constant diameter or transverse dimension extending downwardly from the step to the open end thereof; the wall of said chamber both above and below the said step formed of a single thickness of metal whereby the inner surface thereof is exposed to the flame and the outer surface thereof is in direct contact with the liquid in which same is submerged in operation.
 11. An oil burning combustion unit including an oil atomizer for directing atomized oil and combustion air downwards into a combustion chamber with an opening at the bottom and submerged in a liquid such as water, in which the combustion chamber has in it a step at which its cross-section increases suddenly in a downward direction, the wall of the chamber both above and below the step being formed of a single thickness of metal with its inner surface exposed to the flame and its outer surface immersed in the liquid, the combustion chamber is of circular section and the enlarged portion of it, below the step, has a length between 1/2 and 1 1/2 times its diameter, the combustion chamber also having an upper diverging portion whose diameter increases progressively downwards, a middle cylindrical portion of a substantially constant diameter equal to that of the lower end of the upper portion extending from it to the step, and a lower cylindrical portion of substantially constant diameter extending down from the step to the open end.
 12. An oil burning combustion unit including an oil atomizer for directing atomized oil and combustion air downwards into a combustion chamber with an opening at the bottom and submerged in a liquid such as water, in which the combustion chamber has in it a step at which its cross-section increases suddenly in a downward direction, the wall of the chamber both above and below the step being formed of a single thickness of metal with its inner surface exposed to the flame and its outer surface immersed in the liquid, the combustion chamber is of circular section, the enlarged portion of it, below the step, has a length between 1/2 and 1 1/2 times its diameter, and the opening at the bottom of the combustion chamber is of annular form provided by a plate spaced from the otherwise open lower end of the combustion chamber wall.
 13. An oil burning combustion unit including an oil atomizer for directing atomized oil and combustion air downwards into a combustion chamber with an opening at the bottom and submerged in a liquid such as water, in which the combustion chamber has in it a step at which its cross-section increases suddenly in a downward direction, the wall of the chamber both above and below the step being formed of a single thickness of metal with its inner surface exposed to the flame and its outer surface immersed in the liquid, the combustion chamber is of circular section, the enlarged portion of it, below the step, has a length between 1/2 and 1 1/2 times its diameter, and a pilot gas burner, mounted alongside and having a port connecting it to the combustion chamber, which ignites the flame.
 14. An oil burning combustion unit including an oil atomizer for directing atomized oil and combustion air downwards into a combustion chamber with an opening at the bottom and submerged in a liquid such as water, in which the combustion chamber has in it a step at which its cross-section increases suddenly in a downward direction, the wall of the chamber both above and below the step being formed of a single thickness of metal with its inner surface exposed to the flame and its outer surface immersed in the liquid, the combustion chamber is of circular section, the enlarged portion of it, below the step, has a length between 1/2 and 1 1/2 times its diameter, and the combustion chamber provided with an ultraviolet flame detector which shuts off the supply of oil if the flame is extinguished.
 15. An oil burning combustion unit including an oil atomizer for directing atomized oil and combustion air downwards into a combustion chamber with an opening at the bottom and submerged in a liquid such as water, in which the combustion chamber has in it a step at which its cross-section increases suddenly in a downward direction, the wall of the chamber both above and below the steo being formed of a single thickness of metal with its inner sUrface exposed to the flame and its outer surface immersed in the liquid, and the combustion chamber has an upper diverging portion whose diameter increases progressively downwards, middle cylindrical portion of a substantially constant diameter equal to that of the lower end of the upper portion extending from it to the step, and a lower cylindrical portion of substantially constant diameter extending down from the step to the open end.
 16. An oil burning combustion unit including an oil atomizer for directing atomized oil and combustion air downwards into a combustion chamber with an opening at the bottom and submerged in a liquid such as water, in which the combustion chamber has in it a step at which its cross-section increases suddenly in a downward direction, the wall of the chamber both above and below the step being formed of a single thickness of metal with its inner surface exposed to the flame and its outer surface immersed in the liquid, and the opening at the bottom of the combustion chamber is of annular form provided by a plate spaced from the otherwise open lower end of the combustion chamber wall.
 17. An oil burning combustion unit including an oil atomizer for directing atomized oil and combustion air downwards into a combustion chamber with an opening at the bottom and submerged in a liquid such as water, in which the combustion chamber has in it a step at which its cross-section increases suddenly in a downward direction, the wall of the chamber both above and below the step being formed of a single thickness of metal with its inner surface exposed to the flame and its outer surface immersed in the liquid, and a pilot gas burner, mounted alongside and having a port connecting it to the combustion chamber, which ignites the flame.
 18. An oil burning combustion unit including an oil atomizer for chamber atomized oil and combustion air downwards into a combustion ,hamber with an opening at the bottom and submerged in a liquid such as water, in which the combustion chamber has in it a step at which its cross-section increases suddenly in a downward direction, the wall of the chamber both above and below the step being formed of a single thickness of metal with its inner surface exposed to the flame and its outer surface immersed in the liquid, and the combustion chamber provided with an ultraviolet flame detector which shuts off the supply of oil if the flame is extinguished. 